This invention relates to a piezoelectric filter of the ladder type and more particularly, to an improved piezoelectric filter of the above-described type wherein a plurality of piezoelectric resonators are accommodated in a casing such that it is formed in a compact unit having improved spurious signal characteristics.
Conventionally, a piezoelectric filter of the ladder type is constituted by a plurality of piezoelectric resonators each utilizing a face vibration. As shown in FIG. 1, a typical piezoelectric resonator 4 includes a piezoelectric substrate 1 of a square shape, having its opposing surfaces coated or deposited with electrode layers 2 and 3 of known fabrication. However, when the piezoelectric filter of the above-described type is further arranged to be constructed compactly with the piezoelectric resonator of the square shape, the thickness of the piezoelectric substrate per se has to be much smaller. Such being the case, both the resistance to impact and the resistance to vibration specific to the resultant piezoelectric filter tend to be intolerably low. In addition, since the face shape of the piezoelectric resonator 4 is square, substantial spurious signals are generated in a frequency band of approximately 1.4 times the central frequency of the resultant filter and cannot be avoided. Alternatively, in order to prevent the occurrence of spurious signals, the adoption of a piezoelectric resonator having a circular construction has been proposed. However, when the piezoelectric resonator is formed in a circular construction, with its resonance characteristics being the same as those of a piezoelectric resonator having the square construction, the resultant piezoelectric resonator may inherently involve an increase in size of 10 to 15 percent. Such being the case, as far as the ladder-type filter is concerned, since the number of the piezoelectric resonators is increased with the number of the ladder-stages to be employed, substantially higher manufacturing cost for the piezoelectric substrates or the like cannot be avoided.